Toxoplasma gondii chronically infects approximately 30% of the USA population and causes severe life-threatening Toxoplasmic encephalitis infections in AIDS. Current drug treatments are not well tolerated, and because these treatments have little effect on the slow growing bradyzoite stages, latent parasites remain as a source of recrudescing infection in AIDS. More effective therapies that can target both tachyzoite and bradyzoite stages are urgently needed to treat acute infections as well as to prevent the toxoplasmosis caused by recrudescing parasites. Our preliminary studies have demonstrated that disruption of carbamoyl phosphate synthetase II (CPSII) in the de novo pyrimidine synthesis pathway causes a severe uracil auxotrophy with a corresponding loss of parasite growth and virulence. Consequently the pyrimidine biosynthetic pathway appears to be an excellent target for intervention, and fundamental studies on pyrimidine acquisition have the potential to identify new targets. Due to the need for uridine monophosphate in all life stages, we hypothesize that pyrimidine biosynthesis is necessary to sustain rapid tachyzoite replication as well as the viability of bradyzoites within tissue cysts. We propose to use a combination of genetic, biochemical, and cell biological approaches to better understand the fundamental biology of de novo pyrimidine synthesis and salvage pathways in T. gondii. Inhibitors of pyrimidine biosynthesis will be identified based on a screening strategy of pyrimidine auxotrophy. The information and reagents developed in our studies will be used to validate significant drug targets in the pathway as well as to validate the target(s) of inhibitors identified in this project. These studies will contribute to the development of new therapeutic agents to treat acute infections and may contribute to the development of the first therapeutic agents capable of clearing chronic infection to eliminate reactivation Toxoplasmic encephalitis in AIDS.